Billy Cranston is a fictional character in the Power Rangers universe. He was the original Blue Ranger in the series Mighty Morphin Power Rangers. After this, he continued to serve as an advisor to the Power Rangers team until his departure from the series during Power Rangers: Zeo. He was one of the longest-serving Rangers, behind Thomas Oliver, and Adam Park. He was played by David Yost.

Billy was originally the 'brainy' member of the team, coming up with ideas and gadgets to help the other Rangers. He developed quite a large fan following among certain watchers, and was intensely disliked by other viewers. This was partly due to the over-characterisation, leading Billy to appear weak and clumsy, in stark contrast to the other members of the team. Over time, this was corrected (notably in the alternate-universe Mighty Morphin Power Rangers: The Movie, in which Billy is seen performing backflips). Billy's characterisation was fully-developed by the third season, at which point he no longer wears glasses and appears to be stronger and a more accomplished fighter.

During the Mighty Morphin Alien Rangers miniseries, Billy's role changed. He no longer fought as a Power Ranger directly, but rather acted solely in his advisory role. In fact, Billy was the only Ranger in this period to be portrayed by his own actor - the others had been regressed to children. Billy countered this for himself with a special device that was destroyed before it could be used on the others. He acted in this period as a liason between the Aquitian Rangers and Earth. It was the cure for Billy's reversed aging that eventually served as the plot point that would remove him from the show.

The writers left the choice open as to whether or not Billy would become a Ranger again. He was one of those cited to become the Gold Ranger in place of Trey of Triforia, but the 'negative proton energy' from the Command Centre explosion prevented this.

He did not receive any Ranger powers again, and was written out of the series mid-Power Rangers: Zeo, supposedly because David Yost had asked for more money (Austin St John, Thuy Trang and Walter Emmanuel Jones had been written out of the series for similar reasons in the second season). Billy was last seen in an episode suffering from accelerated aging. He travelled to Aquitar for a cure, and opted to remain there with a female Aquitian he had fallen in love with, Cestria. David Yost had only a small role in Billy's final episode, and his last appearance on the monitor was stock footage.